1. Field of the Invention
This invention relates to microcapsules. More particularly, this invention relates to (i) techniques for loading substances into microcapsules to create an intracapsular reservoir of the substance which is releaseable from the microcapsule at a substantially constant rate over a period of time, and (ii) techniques for using microcapsules as separation and filtration devices.
2. Prior Art
The use of microcapsules containing an intracapsular volume of a substance such as medicinal compounds, biochemical compounds, insecticides, fragrances, and other chemically active substances capable of being released over a period of time is well known. Such compositions are of particular importance in the field of medicine as they have potential for use as bioimplantable or ingestable drug delivery systems.
A critical step in the preparation of such microcapsules is that of loading the capsules with the substance to be released. This step is often quite costly and time consuming. The greatest expense in loading these microcapsules is often incurred as a result of inefficient loading of the substance. In most loading techniques, a relatively large quantity of the substance is needed to load the microcapsules while only a small portion of this substance may actually be loaded, and the remainder may sometimes have to be discarded or further processed before reuse. It is important to avoid waste and inefficient loading of the substance as the substances are often quite expensive.
There are several techniques for the loading of microcapsules with chemically active substances. Perhaps the most common of these techniques is that of liquid-liquid phase separation as disclosed in U.S. Pat. Nos. 3,657,144; 3,664,963; 3,780,195; 4,272,398; and 4,389,330. This technique involves the simultaneous preparation and loading of the capsules. Generally, a substance to be encapsulated and a membrane-forming material are dissolved in a solvent. Next, the solvent, having dissolved substance and membrane-forming material is dispersed in a continuous-phase medium. A portion of the solvent is subsequently evaporated and microcapsules are formed as the membrane-forming material leaves solution and envelopes the substance. Finally the remainder of the solvent is extracted.
Another capsule loading technique is that described by Lim in pending U.S. patent application Ser. No. 485,471, now abandoned. Lim forms capsules having selectively permeable hydrogel membranes which define an aqueous intracapsular volume. These capsules are suspended in a concentrated solution of a substance to be encapsulated within the capsules. The concentration gradient across the membrane causes the substance to diffuse through the membrane. It is generally necessary to soak the capsules one or more times for several hours each time.
U.S. Pat. No. 4,515,736 describes a technique for loading liposomes.
The known capsule loading techniques produce adequate loaded capsules, capable of many end uses. However, the loading process could be substantially shortened and performed less expensively by other techniques. Moreover, it would be useful to develop a loading technique whereby the blank capsules could be conveniently stored and subsequently loaded.